The present invention relates to a method for blowing oxygen in a converter to refine a molten iron and a top-blown lance for blowing oxygen in the converter.
In blowing oxygen into a molten iron in a converter, an oxidation refining is carried out with top-blown oxygen or bottom-blown oxygen mainly for decarburization. In recent years, there is an increased demand for refining a large amount of molten iron in a shorter period of time and achieving a high productivity, than ever before. Further, more oxygen source is required to directly reduce a large amount of iron ore or manganese ore and to melt a large amount of iron scrap in the converter. To this end, a technique, which enables a precise control of composition while blowing a large amount of oxygen stably in a short period of time, is required. Moreover, development of a pretreatment process for the molten iron for the purpose of dephosphorization and desulfurization of the molten iron has drastically reduced the amount of slag generated in the converter refining, and many factors different from those in the conventional process have arisen. To meet such situation, an immediate optimization of the oxygen blowing method in the converter is now an urgent matter.
In the oxidation refining with the top-blown lance, the oxygen is supplied from a divergent nozzle, known as Laval nozzle, installed on a tip of the top-blown lance into the converter as a supersonic or a subsonic jet. In this case, a shape of the Laval nozzle is designed generally depending on the refining conditions in a high carbon region from the beginning to the middle of the blow process in which comparatively much oxygen is supplied to prevent a decline of efficiency of reactions such as the decarburization reaction. Hereinafter, the amount of the supplied oxygen is referred to as xe2x80x9coxygen-flow-rate.xe2x80x9d In Mother words, in case of the high oxygen-flow-rate, the blown oxygen is expanded properly to be supersonic-like by the Laval nozzle, on the contrary, in case of the low oxygen-flow-rate, corresponding to the low carbon region in the end of the blow, the oxygen expands excessively within the Laval nozzle, resulting in keeping the oxygen from being supersonic-like. In the high carbon region from the beginning to the middle of the blow, molten pool contains over about 0.6 mass % of C, and in the low carbon region in the end of the blow, the molten pool contains about 0.6 mass % or less of C.
When the Laval nozzle based on such design concept is applied to the oxygen blowing method having the still higher oxygen-flow-rate aiming to achieve a high productivity, a jet flow velocity of the oxygen jet supplied from the top-blown lance is further increased, the flow velocity of the jet reaching a surface of the molten pool within the converter is increased and a surface of the molten metal fluctuates more vigorously. In the conventional blow with large amount of the slag of more than 50 kg per ton of molten steel, this design concept was crucial to ensure the oxygen jet to penetrate through the slag layer.
However, in the blow with a small amount of the slag such as those in recent days, such design concept becomes less necessary, contrarily, in the blow with a small amount of slag, the fluctuation of the surface of the pool accompanying the increase of the jet flow velocity causes vigorous scatter of the molten pool including spitting and splashing and increases metal adhesion to regions such as a throat and a hood, the top-blown lance, and equipment for off gas besides, thereby affects adversely on operation and causes a waning productivity due to the decline of yield of iron. Moreover, iron dust increases significantly with the scatter, leading to a decline of the yield of iron also from a viewpoint of the dust.
To restrain such deterioration of the operating conditions, a number of measures, in which the operation conditions including a distance between the tip of the top-blown lance and a bath surface and the oxygen-flow-rate are controlled, have been proposed, with hardware of the top-blown lance including a hole size and bevel of the Laval nozzle being optimized. Hereinafter, the distance between the tip of the top-blown lance and the bath surface is written as xe2x80x9clance-height xe2x80x9d. For example, JP-A-6-228624 discloses the blow method in which the shape of the top-blown lance is optimized, and the oxygen-flow-rate and the lance-height are controlled within a proper range adapted for the shape of the Laval nozzle. However, if a structure of the Laval nozzle and the lance-height are altered to restrain the scatter of iron and the dust during the increased flow as described in that number of the publication, a trace and geometry of the oxygen jet brown out from the top-blown lance are extremely changed, therefore secondary adverse affects, such as an unnecessary post combustion and the decline of the reaction efficiency due to the fluctuation of the reaction interface area, occur. Moreover, if the alteration of the lance-height and the like are hard physically or operationally, the measure cannot be advantageous.
On the other hand, in the low carbon region in the end of the blow, since the supplied oxygen is also consumed in the oxidization of the iron as well as the decarburization, the oxygen-flow-rate is reduced to restrain the oxidization of the iron and improve the oxygen efficiency for the decarburization. In this case, the oxygen-flow-rate greatly deflects downward from an optimum flow value of the Laval nozzle, therefore maximum effect of the Laval nozzle cannot be obtained, and the oxygen jet is attenuated unnecessarily, resulting in the decline of the efficiency of the decarburization in the end of the blow, as indicated in increased T.Fe in the slag. Moreover, although the oxygen-flow-rate must be controlled in extremely low order in the end of the blow in order to improve a hitting accuracy of the composition at the endpoint of the blow, an excessively low order of the rate extremely reduces dynamic pressure of the oxygen and causes rapid oxidization of the iron, therefore the oxygen-flow-rate has its limit in reduction. It is noted that the T.Fe is a total value of the iron content in all of the iron oxides including FeO and Fe2O3 in the slag.
Japanese unexamined patent publication No.10-30110 discloses the oxygen blowing method which employs the top-blown lances having an exit diameter from 0.85 D to 0.94 D in the high carbon region and the exit diameter from 0.96 D to 1.15 D in the low carbon region respectively, to an optimum expansion exit diameter D of the Laval nozzle determined from the throat diameter of the Laval nozzle and the oxygen-flow-rate. The Publication also describes that even when the same Laval nozzle is used, the exit diameter can be adjusted satisfying the above described range to the optimum expansion exit diameter D by altering the oxygen-flow-rate and a back pressure of the Laval nozzle P.
In Japanese unexamined patent publication No.10-30110, it is described that a soft blow can be achieved in the high carbon region, and a hard blow can be achieved in the low carbon region by altering the shape of the Laval nozzle as above, and the reduction of the dust and the reduction of iron oxidization can be achieved at the same time. However, in this blow method, two or more types of the top-blown lances, each lance having different shape, must be used to control the refining surely, and certain complexity in equipment and operation can not be disregarded. In addition, when the same single top-blown lance is used, some problems may occur, that is, design of the Laval nozzle becomes complicated, and the oxygen-flow-rate cannot be altered freely depending on the conditions within the converter. Moreover, for an application in the minimum amount of the slag, many unclear points still remain.
It is an object of the present invention to provide an oxygen blowing method in a converter wherein the scatter of the iron and the generation of the dust are reduced at the high-oxygen-flow-rate period in the high carbon region as a peak of the decarburization, the oxidization of the iron is restrained at the low-oxygen-flow-rate period in the end of the oxygen blowing, and the reaction is stably performed at the low oxygen-flow-rate.
To achieve the object, the present invention provides an oxygen blowing method in a converter, which uses a top-blown lance having a Laval nozzle installed at the tip of the top-blown lance.
The Laval nozzle has a back pressure of the nozzle Po(kPa) satisfying the following formula with respect to the oxygen-flow-rate Fhs(Nm3/hr) per hole of the Laval nozzle, determined from the oxygen-flow-rate Fs(Nm3/hr) in a high carbon region as a peak of the decarburization, and a throat diameter Dt(mm).
Po=Fhs/(0.00465xc2x7Dt2) 
An exit diameter De of the Laval nozzle satisfies the following formula with respect to the back pressure of the nozzle Po(kPa), an ambient pressure Pe(kPa), and the throat diameter Dt(mm).
De2xe2x89xa60.23xc3x97Dt2/{(Pe/Po)5/7xc3x97[1xe2x88x92(Pe/Po)2/7]1/2}
It is preferable in the oxygen blowing method that the exit diameter De of the Laval nozzle satisfies the following formula with respect to the back pressure of the nozzle Po(kPa), the ambient pressure Pe(kPa), and the throat diameter Dt(mm).
De2xe2x89xa60.185xc3x97Dt2/{(Pe/Po)5/7xc3x97[1xe2x88x92(Pe/Po)2/7]1/2}
Further, it is more preferable that the exit diameter De of the Laval nozzle satisfies the following formula with respect to the back pressure of the nozzle Po(kPa), the ambient pressure Pe(kPa), and the throat diameter Dt(mm).
0.15xc3x97Dt2/{(Pe/Po)5/7xc3x97[1xe2x88x92(Pe/Po)2/7]1/2}xe2x89xa6De2xe2x89xa60.18xc3x97Dt2/{(Pe/Po)5/7xc3x97[1xe2x88x92(Pe/Po)2/7]1/2}
In the oxygen blowing method, the top-blown lance has multiple Laval nozzles, and at least one of those Laval nozzles is required to satisfy conditions of the following two formulas.
Po=Fhs/(0.00465xc2x7Dt2) 
De2xe2x89xa60.23xc3x97Dt2/{(Pe/Po)5/7xc3x97[1xe2x88x92(Pe/Po)2/7]1/2}
More preferably, the conditions of the following two formulas are satisfied.
Po=Fhs/(0.00465xc2x7Dt2) 
De2xe2x89xa60.185xc3x97Dt2/{(Pe/Po)5/7xc3x97[1xe2x88x92(Pe/Po)2/7]1/2}
In the oxygen blowing method, it is preferable that the oxygen blowing is carried out at the amount of the slag of less than 50 kg per ton of the molten steel. More preferably, the amount is less than 30 kg per ton of the molten steel.
Moreover, in the oxygen blowing method, the Laval nozzle has the back pressure of the nozzle Poo(kPa), satisfying the following formula with respect to the oxygen-flow-rate FhM (NM3/hr) per hole of the Laval nozzle determined from the oxygen-flow-rate FM(Nm3/hr) in the low carbon region in the end of the blow, and the throat diameter Dt (mm).
Poo=FhM/(0.00465xc2x7Dt2) 
It is desirable that the exit diameter De has a ratio (De/Deo) of 1.10 or less to the optimum exit diameter Deo(mm) which is given from the back pressure Poo(kPa), the ambient pressure Pe(kPa), and the throat diameter Dt(mm) according to the following formula.
Deo2=0.259xc3x97Dt2/{(Pe/Poo)5/7xc3x97[1xe2x88x92(Pe/Poo)2/7]1/2}
Further, this invention provides the oxygen blowing method that blows using the top-blown lance having the Laval nozzle installed on its tip.
The Laval nozzle has the back pressure of the nozzle Poo(kPa) satisfying the following formula with respect to the oxygen-flow-rate FhM(Nm3/hr) per hole of the Laval nozzle determined from the oxygen-flow-rate FM(NM3/hr) in the low carbon region in the end of the blow, and the throat diameter Dt(mm).
Poo=FhM/(0.00465xc2x7Dt2) 
The exit diameter De of the Laval nozzle has the ratio (De/Deo) of 0.95 or less to the optimum exit diameter Deo(mm) which is given from the back pressure Poo(kPa), the ambient pressure Pe(kPa), and the throat diameter Dt(mm) according to the following formula.
Deo2=0.259xc3x97Dt2/{(Pe/Poo)5/7xc3x97[1xe2x88x92(Pe/Poo)2/7]1/2}
In the oxygen blowing method, the top-blown lance has the multiple Laval nozzles, and at least one of those Laval nozzles is required to satisfy the conditions of the following two formulas.
Poo=FhM/(0.00465xc2x7Dt2) 
De2=0.259xc3x97Dt2/{(Pe/Poo)5/7xc3x97[1xe2x88x92(Pe/Poo)2/7]1/2}
In the oxygen blowing method, it is preferable that the oxygen blowing is done at the amount of the slag of less than 50 kg per ton of the molten steel. More preferably, the amount is less than 30 kg per ton of the molten steel.
Further, the present invention provides a top-blown lance for blowing oxygen having the Laval nozzle installed on its tip.
The Laval nozzle has the back pressure of the nozzle Po(kPa) satisfying the following formula with respect to the oxygen-flow-rate Fhs(Nm3/hr) per hole of the Laval nozzle determined from the oxygen-flow-rate Fs(Nm3/hr) in the high carbon region as the peak of the decarburization, and the throat diameter Dt(mm).
Po=Fhs/(0.00465xc2x7Dt2) 
The exit diameter De of the Laval nozzle satisfies the following formula with respect to the back pressure of the nozzle Po(kPa), the ambient pressure Pe(kPa), and the throat diameter Dt(mm).
De2=0.23xc3x97Dt2/{(Pe/Poo)5/7xc3x97[1xe2x88x92(Pe/Poo)2/7]1/2}
Further, the present invention provides the top blown lance for blowing oxygen having the Laval nozzle installed on its tip.
The Laval nozzle has the back pressure of the nozzle Poo(kPa) satisfying the following formula with respect to the oxygen-flow-rate FhM(Nm3/hr) per hole of the Laval nozzle determined from the oxygen-flow-rate FM(Nm3/hr) in the low carbon region in the end of the blow, and the throat diameter Dt(mm).
Poo=FhM/(0.00465xc2x7Dt2) 
The exit diameter De of the Laval nozzle has the ratio (De/Deo) of 0.95 or less to the optimum exit diameter Deo(mm) which is given from the back pressure of the nozzle Poo(kPa), the ambient pressure Pe(kPa), and the throat diameter Dt(mm) according to the following formula.
Deo2=0.259xc3x97Dt2/{(Pe/Poo)5/7xc3x97[1xe2x88x92(Pe/Poo)2/7]1/2}